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6.5 ppm

chemical and chemical processing industries for the separation of close-boiling, pinched, or azeotropic systems for which simple distillation is either too expensive or impossible. With an azeotropic feed mixture, presence of the azeotroping agent results in the formation of a more favorable azeotropic pattern for the desired separation. For a close-boiling or pinched feed mixture, the azeotroping agent changes the dimensionality of the system and allows separation to occur along a less-pinched path. Within the general heading of azeotropic distillation techniques, several approaches have been followed in devising azeotropic distillation flowsheets including:

1. Choosing an entrainer to give a residue curve map with specific distillation regions and node temperatures.

2. Exploiting changes in azeotropic composition with total system pressure.

3. Exploiting curvature of distillation region boundaries.

4. Choosing an entrainer to cause azeotrope formation in combination with liquid-liquid immiscibility.

The first three of these are solely VLE-based approaches, involving a series of simple distillation operations and recycles. The final approach also relies on distillation (VLE), but also exploits another physical phenomena, liquid-liquid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-18.

Methyl Propionate

Methyl Propionate

Batch Distillation Curve

FIG. 13-63 Batch distillation paths. (a) Methanol-methyl propionate-water system.

FIG. 13-63 Batch distillation paths. (a) Methanol-methyl propionate-water system.

System

Type

Entrainer(s)

Remark

Exploitation of homogeneous azeotropes

No known industrial examples

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